Laser ablation has proven a useful tool for micrometer scale machining of metal and dielectric materials. The laser ablation process is not completely understood, however, it generally can be divided into two types: a thermally activated ablation associated with nanosecond timescale laser pulses; and a non-thermal ablation associated with picosecond and femtosecond pulse lengths. The thermal vaporization is also known as “strong ablation,” since it results in greater rates of material removal and is the dominant mechanism at laser fluences well above the ablation threshold. The non-thermal ablation is also known as “gentle ablation,” since it occurs near the ablation threshold and tends to result in optimal surface quality with material removal controllable on the nanometer scale. For example, the gentle ablation does not produce a melt zone. As such, the gentle ablation mechanism is more desirable for precise control of the machining process than is thermal ablation. Gentle ablation is thought to occur through a physical mechanism analogous to the “Coulomb explosion” seen in gas clusters, wherein a laser pulse is used to rapidly evacuate electrons from a region of the gas such that the Coulombic repulsion of the positive ions then leads to rapid expansion. In solids, it is hypothesized Coulomb explosion is driven by positively charged near-surface layers which electrostaticly repel each other, possibly aided by the pull of ionizing electrons [Gamaly, 2002].
The key to activating this non-thermal, or “cold” ablation then lies in the electron dynamics. Work in the prior art has entailed modeling the electron dynamics with the Fokker-Planck equation. The production of electrons is due to two types of optical absorption: the sum over all interband optical absorptions including multi-photon absorptions; and the “avalanche” photoionization. Avalanche photoionization is due to free carrier absorption and therefore of primary importance in metals. It also can become dominant in dielectrics once the carrier density reaches a critical value. To see this point, it is useful to consider the differential equation conventionally taken to describe the carrier dynamics a laser field:dNe/dt=ΣαmIm+βNeI−Ne/τ,where Ne is the carrier density, I is the laser intensity inside the sample, αm are the multi-photon absorption coefficients, β is the “avalanche” absorption coefficient, and τ is the phenomenological carrier relaxation time [Mero, 2005]. For purposes of understanding the carrier production during ultra-short laser pulses, the relaxation term may be neglected. The equation may then be viewed as an interpolation scheme—during the leading edge of the laser pulse, the carrier density is small and therefore the avalanche absorption will be negligible, resulting in a carrier density Ne≈∫ΣαmIm dt, whereas later in the pulse, once the carrier density becomes significant, the avalanche term becomes dominant and the solution takes the exponential form Ne≈Noexp{βIt} [Gamaly, 2002]. For dielectric materials irradiated by a typical femtosecond laser wavelength of ˜800 nm, the linear optical absorption will be small. Therefore, the avalanche seeding process proceeds through multi-photon absorption, and hence requires strong laser intensity. Once the seed density is reached the avalanche absorption leads to very rapid ablation. Thus, the seed carrier density typically invoked as the ablation threshold density is the critical density at which the plasma becomes opaque. This condition occurs when ωp≧ω, where ωp is the Drude plasma frequency and ω is the laser photon frequency. The plasma frequency and photon frequency are given by the relations ωp2=Nee2/εom*, and ω=2πc/λ, respectively, where e is the electronic charge, εo is the vacuum permittivity (8.85×10−12 C2/N·m2), m* is the electron effective mass, c is the speed of light, and λ is the laser wavelength. The critical Drude plasma density is then: Ncr=4m*εoπ2c2/e2λ2, and typically takes values of Ncr≈1021/cm3. Note this not the actual ablation density, but rather, the density which seeds the exponential avalanche process.
To estimate the charge density needed to induce ablation through the Coulomb explosion, it has been suggested ablation occurs whenever the electrostatic force due to the surface charge density exceeds the bonding force [Bulgakova, 2004]. The bonding stress may be estimated as a percentage of the Young's modulus, typically taken at ˜5-10%. This implies electric fields of ˜1-5×1010 V/m are required to exceed the bonding force. The surface charge densities corresponding to such fields can be estimated from the Poisson relation: F=(2 eNeV/ε)1/2, where V is the built-in surface voltage (˜1V) and ε is the permittivity of the material (≈10εo). From this expression, it may be seen the surface charge densities required to produce such fields are greater than 1×1022/cm3. The threshold fractional ionization per atom required for Coulomb explosion of silicon has been estimated in the range 0.3-0.65, corresponding to charge densities of 1.5-3.5×1022/cm3 [Stoian, 2004].
The Coulomb explosion mechanism has been clearly observed in laser ablation of dielectrics [Stoian, 2000]. It has also been postulated that low conductivity silicon behaves similarly to dielectrics, thus allowing Coulomb explosion [Roeterdink, 2003]. However, calculations have been performed suggesting the threshold charge density is not reached due to charge reneutralization [Bulgakova, 2004]. A fundamental advantage of the disclosure contained herein is the use of strong linear semiconductor absorption such that the conditions for Coulomb explosion are met without requirement for avalanche photoionization. This enables the use of a lower laser intensity to ablate material, hence providing enhanced control over material removal rates attained in the Coulomb explosion (CE) process. A further advantage may be obtained over the conventional CE machining process by the function of the static electric field to enhance the linear optical absorption. This dependence of the optical absorption on electric field has never been disclosed or utilized in the prior art of laser ablation. In particular, nearby to strong interband transitions, the optical absorption takes the form α(F)=αo+α2F2, where αo is the linear optical absorption at zero field, and α2 is a third order non-linear optical absorption coefficient. α2 itself depends on the electric field and in particular describes the redshifting of semiconductor interband transitions in a strong electric field [Keldysh, 1958]. The field dependence of α2 may be used to moderate the laser absorption by an order of magnitude or more. Therefore, given selection of laser wavelength to coincide with strong optical absorptions such that avalanche photoionization is suppressed or takes secondary importance, the addition of a strong electric field may be utilized to further reduce the intensity required to generate the threshold carrier density.
Additionally, conventional laser ablation cannot be conveniently used for nanometer scale machining since it is limited by far field spot size focus limit, typically on the order of a micron. By the use of nanometer scale surface variations to provide nanometer scale variation of the electric field, ablation can be achieved on lateral scales smaller than the laser spot diameter.
Thus, while the prior art may be suitable for the particular purposes which they address, they are not as suitable for laser ablation of semiconductor materials. In these respects, the laser ablation technique according to the present disclosure substantially departs from the conventional concepts and designs of the prior art, and in so doing provides a technique primarily developed for the purpose of nanoscale laser machining of semiconductor materials.